Vehicular drive system with electric assist

ABSTRACT

A vehicle is equipped with a main drive system and an electric assist drive system which is actuated by the operator when the vehicle speed falls below some predetermined speed. A prime mover for the vehicle, in addition to driving two wheels directly through a primary transmission, drives a three-phase electrical alternator. The alternator energizes an induction motor with a torque-regulating control system for independently driving two other wheels of the vehicle to provide greater traction at low speeds. The rotor circuit of the induction motor includes a rectifier bridge which feeds a line-commutated inverter for coupling the slip power back to the stator input. The control system determines the firing angle for conduction of the switches in the inverter to control the output torque of the induction motor as a function of speed. The system includes a shaping network which defines the desired speed-torque characteristic for accelerating the vehicle; and it generates a signal representative of a demand torque for a given motor speed. Another circuit sensing rotor current generates a signal representative of the load torque. A comparison circuit receives the two signals representative respectively of the actual torque and the desired torque for that speed; and it generates an error signal for controlling the firing angle of the switches in the inverter circuit such that the motor and vehicle are accelerated along the torque-speed envelope of the shaping network once the assisting system is actuated. Thus, the control system regulates the output torque of the assisting induction motor as a predetermined function of vehicle speed.

United States Patent [72] Inventors Lee T. Magnuson Davenport, Iowa;Alexander Kusko, Newton Centre, Mass. [21 Appl. No. 730,541 [22] FiledMay 20, 1968 [45] Patented Feb. 9, 1971 [54] VEHICULAR DRIVE SYSTEM WITHELECTRIC Primary ExaminerBanjamin Hersh Assistant Examiner-Milton L.Smith Attorney-Dawson, Tilton, Fallon & Lungmus ABSTRACT: A vehicle isequipped with a main drive system and an electric assist drive systemwhich is actuated by the operator when the vehicle speed falls belowsome predetermined speed. A prime mover for the vehicle, in addition todriving two wheels directly through a primary transmission, drives athree-phase electrical alternator. The alternator energizes an inductionmotor with a torque-regulating control system for independently drivingtwo other wheels of the vehicle to provide greater traction at lowspeeds. The rotor circuit of the induction motor includes a rectifierbridge which feeds a line-commutated inverter for coupling the slippower back to the stator input. The control system determines the firingangle for conduction of the switches in the inverter to control theoutput torque of the induction motor as a function of speed. The systemincludes a shaping network which defines the desired speed-torquecharacteristic for accelerating the vehicle; and it generates a signalrepresentative of a demand torque for a given motor speed. Anothercircuit sensing rotor current generates a signal representative of theload torque. A comparison circuit receives the two signalsrepresentative respectively of the actual torque and the desired torquefor that speed; and it generates an error signal for controlling thefiring angle of the switches in the inverter circuit such that the motorand vehicle are accelerated along the torque-speed envelope of theshaping network once the assisting system is actuated. Thus, the controlsystem regulates the output torque of the assisting induction motor as apredetermined function of vehicle speed.

PATENTEDFEB 919m R 3,561,557

' sum 1 [IF 4 MAX. TORQUE ELECTRIC ASS/ST (LOADED) CONSTANT HORsE POwERMAX. TOROuE ELECTRIC A ss/sT(EMPTY) MAX. TORQUE 2 WHEEL OR/v (LOADED)NAx TORauE 2 WHEEL DRIVE (EMPTY) ENGAGE ASSIST BEFORE LOADING CONSTANTHORsEPOwER 24 F/GS. SPEED RECTIFIER BRIDGE 34 35 INVE R T Ef "l i PRIORART 33/ w m In PHAsE k INPU T2 Y :3 I I I J I .Tlrmlvvw [vu 3 [H 36PHA$E2- FIRING INPUT 'fi c/Rcu/T F/G. 4. F/ 3A O V 3'0 6O 120135150180INVENTORS I LEE 7'. MAGNUS ON ROTOR ALEXANDER KUSKO cuRRENT M MAE, t azREcT/F/ER INVERTER I -REG/ON REGION ATTYS PATENTEUFEB 9|97| A 3.561.557,

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TORQUE SLIP (SPEED) I 20 Y 33; 35 INDUCTION 3 PHASE INVERTER ALTENATORMOTOR REcT/F/ER c/RcU/T T W56 g /48 7 /5O Zfic fiff /gE E z gg TORQUE TO[54 VOLTAGE CON vER TER CONVERTER CON TROL AMRL/F/ER DEMAND 49 T 54ATORQUE NET WORK START cOMRAR/sON c/RcU/T 'R c/RcU/T i /E Q) cURRENTACTUATOR @SHAP/NG NETWORK LIMIT @TORQU CONTRSL INVENTORS LEE T. MAGNUSONFIG. 7' ALEfdANDER KUSKO ATTYs,

VEI-IICULAR DRIVE SYSTEM WITH ELECTRIC ASSIST BACKGROUND components andthe inherent torque limit of the motor. The

amount of power drawn from a utility supply line is usually not a majorconsideration. The situation is different for a vehicle drive systemwherein the only power source is a prime mover such as a gas turbineengine. The maximum horsepower which the motor can transmit to thewheels is limited by the capability of the prime mover.

Typically, however, especially at low speeds, a vehicle drive system istraction limited (that is, although additional horsepower is available,it is not delivered due to wheel slip); at the higher speeds, it ishorsepower limited.

Many vehicles such as earthmoving scrapers carry nearly fifty percent ofthe load on unpowered wheels. The remainder of the vehicle weight issupported by wheels which are powered by a conventional mechanicaltransmission which is herein sometimes referred to as the primarytransmission. Systems have been built in which an auxiliary engine andtransmission is used to drive the previously unpowered wheels; however,in such a system synchronization between the primary and the auxiliaryengines and transmissions is usually attained by their gear ratios; andthis represents a significant problem in synchronizing the engines torun at the proper speeds.

SUMMARY In the present invention, an electric assist transmission powersthe previously unpowered wheels of a vehicle. The electric assisttransmission is selectively activated by an operator during high drawbarpull conditionsthat is, when the primary transmission is torque-limitedby the traction limit of the wheels driven by the primary transmission.When activated, the electric assist accelerates the electric motor andthe vehicle as predetermined by the envelope of the speed-torquecharacteristic stored in the shaping network which may vary fromapplication to application. The electric assist is automaticallydisengaged when it is possible to transmit all of the available enginehorsepower through the primary transmission (i.e. when the vehicle hasreached sufficient 5 speed so that excessive slip does not occur).

The electric assist system includes an alternator driven by the primemover, a wound-rotor induction motor for driving the two rear wheels, aninverter for receiving rectified rotor current and coupling it back tothe stator input, and a control system for controlling the inverterswitches such that the output torque of the electric motor is apredetermined function of vehicle speed.

The system nearly doubles the maximum drawbar pull capacity of thevehicle, and makes it possible to utilize more engine horsepower at lowvehicle speeds.

As distinguished from other multiple transmission vehicles, such aslocomotives, the present assist system does not operate continuously,but only over the low speed range when actuated by an operator. Thepresent system has advantages over those which use DC motors in thatmechanical commutation is not a problem; and thus, the motor can operateat higher speeds and therefore be smaller. The present system hasadvantages over systems for speed control of squirrel-cage inductionmotors in the simplicity of its control circuitry. Further, more thanone auxiliary transmission can be used in accordance with the invention.

Further objects and advantages of the present invention will be apparentto persons skilled in the art from the following detailed descriptionaccompanied by the attached drawing illustrating a preferred embodimentof the invention.

DRAWING FIG. 1 is a schematic plan view of an earthmoving scraperincorporating the present invention;

FIG. 2 illustrates the speed-torque characteristic of the vehicle ofFIG. 1;

FIGS. 3 and 3A schematically illustrate a system having an inductionmotor with a line-commutated inverter in its rotor circuit and thefiring circuit therefor;

FIG. 4 is a plot of firing angle vs. applied voltage for a commutatingswitch;

FIG. 5 illustrates various voltage and current wavefonns for theinverter of FIG. 3;

FIG. 6 is a plot of speed-torque characteristics for the system of FIG.3 with a superimposed speed-torque demand characteristic incorporated bythe present invention;

FIG. 7 is a functional block schematic diagram of an electric assistcontrol system according to the present invention; and

FIGS. 8A-8C are detailed circuit schematic diagrams of the controlsystem of FIG. 7.

DETAILED DESCRIPTION Although it is not so limited, the presentinvention is preferably used to assist the main drive system for anearthmoving scraper. In order to better understand the invention,reference is made to FIG. 1 showing a scraper adapted to incorporate theinvention. The principal elements of the scraper with which the presentinvention are concerned are a tractor generally designated 10, and anelevating scraper generally designated II. The tractor I0 is equippedwith a prime mover 12, which may be a conventional gas turbine en gine,and which drives a primary transmission 13 for applying traction towheels 14. The transmission I3 may be a conventional power shifttransmission.

The scraper 11 is provided with a bowl 15 for receiving earth and aplurality of flights 16 for performing the actual scraping operation.The flights 16 are powered by a hydraulic motor 17 which, in turn, isenergized by the prime mover l2. Earthmoving scrapers including theabove-described elements are known and commercially available.

The present invention contemplates that the prime mover 12, in additionto driving the primary transmission 13, also drive an alternator 19 toenergize an induction motor 20 mounted above the rear wheels 21 on whichmost of the weight of the scraper II is carried. The electric assist(that is, the energizing of the induction motor 20) is designedprimarily to be called into use at the low speed range for the vehiclewhereby it becomes a four-wheel drive system. The improvedcharacteristic of the type of the system of FIG. 1 is shown in FIG. 2.

In FIG. 2, the heavier curve 24 is a rectangular hyperbolarepresentative of an idealized constant horsepower output of the primemover 12. The dashed line 25 represents the maximum torque capable ofbeing transmitted to the wheels 14 when the bowl 15 is empty; and thesolid line 26 represents the maximum torque for the two-wheel drivesystem when the bowl is loaded. A solid line 27 joining the line 26 withthe curve 24 represents a portion of the speed-torque characteristic ofa vehicle under various loads. Hence, the characteristics formed by thelines 26 and 27 together with the lower portion of the constanthorsepower curve 24 define the boundary for a speed-torque range for thetwo-wheel drive system.

The present invention contemplates engaging or actuating the electricassist at some speed, for example, the speed represented by the point 28(the low traction limit speed) on the curve 24 to apply additionaltorque to the previously undriven wheels 21. In this situation, thedashed line 29 represents the maximum torque for the electric assistsystem with an empty scraper; the line 30 represents the maximum torquefor the electric assist system when the scraper is loaded, and theareabetween the solid line 30 and line 26 represents the operationregion for the system. In other words, the constant horsepower curve 24is extended up to the line 30 to define the range over which theimproved system may operate. There will, of course, be some maximumspeed beyond which the primary transmission 13 may not be driven;however, this does not concern the present invention.

The motor 20 of FIG. 1 is, a wound-rotor induction motor with a linecommutated-inverter in its rotor circuit. Induction motor systems ofthis kind for constant torque output are known; one wiring schematic forsuch a system is illustrated in FIG. 3.

In FIG. 3, a three-phase voltage source is coupled directly to thestator winding of the motor 20; and the rotor windings are brought outthrough the slipring assembly generally designated 32. A three-phaserectifier bridge, generally designated 33 and enclosed within dashedline in FIG. 3. receives the rotor current flowing through the sliprings32 and generates a DC rotor current flowing through an inductor 34 inthe direction of the arrow into the positive terminal of an invertercircuit 35 (also enclosed within dashed line). I

The inverter 35 is a three-phase inverted connected in circuit accordingto known technology; and it includes six silicon controlled rectifiersor thyristors, which term herein is to be taken broadly so as to includeall equivalents. The three output leads of the inverter 35 are coupleddirectly back to the appropriate phase of the source which feeds thestator of the motor 20. The firing order of thethyristors of theinverter 35 is identified by a roman numeral (lVl) associated with eachone; and a firing circuit 36 (FIG. 3A) having six separate output leads(one connected respectively to the gate terminals of each of thethyristors in the inverter 35) determines the firing order and timing ofthe inverter 35.

It will be helpful in understanding the present invention to realizecertain characteristics of a wound-rotor induction motor system with aIine-commutated inverter. The speed of an induction motor may be variedby varying the frequency of the stator supply voltage, by varyingresistance in the rotor circuit, or by generating a back emf in therotor circuit. The torque-speed characteristic of an induction motor inwhich the rotor resistance is increased has the effect of decreasing theslope of the operating characteristic over the operating range (see thecurve 40' in FIG. 6). This type of control is inefficient because of thepower loss in the rotor resistance.

In a line-commutated inverter control system, the inverter couples theslip power back to the AC supply line by adjusting the firing angle ofthe switches in the inverter. The commuta tion problem is not asignificant one; and the motor always operates at the same inputfrequency thereby obviating the need to vary the input voltage.-

The firing circuit 36 provides a source for predetermining the phase(relative to some time base supplied by the primary source) of thefiring angle for thethyristors. Hence, the firing circuit 36 alsoreceives an input from the three-phase primary source. The DC terminalvoltage of the inverter at a fixed AC voltage is proportional to thecosine of the firing angle, a:

V,= 1.35 a) (l) where, V, is the DC input terminal voltage to theinverter;

V is the line-to-line output voltage; and

a is the firing or conduction angle for the thyristors.

It will be noted that the firing angle, a, is customarily measured fromthe point at which the line-to-line voltage is 60, so that a correspondsto the normal full rectifier output voltage.

FIG. 4 shows the inverter input voltage, V,-, as a function of thefiring angle of the switches; and it will be observed that for apositive input voltage (that is, the angle a is between 0 and 90) thenetwork acts as a rectifier, whereas when a is in the range between 90and 180, the network is properly in the inverter region.

When the firing angle is 180 and the DC input voltage is 1.35 times theRMS line-to-line voltage, there is no output current from the inverterto the AC line; and as the firing angle or decreases from 180, currentbegins to flow through the inverter back to the AC line. Hence, asillustrated by the horizontal arrow in FIG. 4, a decrease in a generatesan increase in rotor current and output torque.

Turning now to FIG. 5, there is shownthree idealized line diagramsillustrating the voltage waveforms of thyristor l for the case when a isThe first line of FIG. 5 shows the voltage wavefonns for the threesupply lines. As illustrated, the angle a is measured from a point atwhich the voltage from line 1 to line 2 is at 60". For reliable inverteroperation, the minimum extinction angle B, must be such as to providethe negative voltage on thyristor I at least for sufficient time toaccomplish complete commutation of all possible AC linevoltagedisturbances and the maximum load current. In line 2 of FIG. 5, E,represents the. average voltage output from the rectifier bridge 33 ofFIG. 3; and the sequence of smaller numbers at the top line 2 indicatesthe individual thyristors of the inverter 35 which are then conducting.

The waveform designated by reference numeral 38 is the reverse voltageacross the input terminals to the inverter 35. In line 3 of FIG. 5, thewaveform designated by reference numeral 39 is representative of thecurrent flowing in thyristor I. Once a thyristor is fired, it remains ina conducting state for 60, and each thyristor conducts for twoconsecutive 60 intervals for a total of l20. Additional informationconcerning the design and operation of inverterv circuits may be foundin a book entitled PRINCIPLES OF INVERTER CIRCUITS, by Bedford andI'Ioft, John Wiley & Sons, Inc. New York, 1964- particularly in Chapter3.

Turning now to FIG. 6, there is shown a family of characteristicspeed-torque (slip speed in r.p.m. is equal to the synchronous speedminus the actual speed) curves for various values of cos a. Theindividual curves are designated 40, 41. 42 and 43 respectively; and forthe curve 40, cos a 0; for the curve 41, cos a -0.25; for the curve 42,cos a 0.5; and for the curve 43, cos a O.'75. The curves illustrated arefor a condition in which the rotor resistance is minimum. The effect ofincreased rotor resistance, as already indicated, for a constant cos a,is to change the characteristic curve 40 to that illustrated at 40'.

Turning now to FIG. 7, there is shown a block schematic diagram of theinventive control system. The previously mentioned alternator l9,induction motor 20, rectifier 33, and inverter 35 are illustrated inblock form with corresponding reference numerals. Although only a singleline is shown in the interconnections of the blocks 19, 20, 33, and 35,it will be appreciated that each of these shown lines represents threeseparate electrical feed lines as shown in FIG. 3.

The control system, in its broader aspects has three principal elements.One is what is sometimes herein referred to as the demand-torque orshaping network which generates a signal representative of apredetermined-desirable (i.e. demand) torque for the actual speed of themotor. As will be explained in greater detail within, the shapingnetwork may be thought of as a function generator which stores aspeed-torque envelope to control the motor in the sense that for anygiven motor speed, it will generate a corresponding signalrepresentative of a desired torque for that speed.

A second major element of the control circuitry generates a signalrepresentative of the actual torque being produced by the motor. Anerror signal is generated which is representative of the differencebetween the desired torque signal and the ac tual torque signal.

A third principal element of the control circuitry sets the motor torqueequal to the desired torque for a given speed as indicated by thespeed-torque characteristic stored in tht shaping network. This isaccomplished by setting the firing angle for the thyristors of theinverter according to this CI'IO! signal. For example, if the desiredtorque is very much greatei than the actual torque being'transmitted tothe rotor, then tht error signal will be such as to cause the firingangle to shif toward the 90 mark; whereas, if the actual torque beinrproduced is larger than the desired torque, then the erroi signal willbe such as to cause the firing angle to shift towart mark, representinga cutoff condition. When the desirer torque and actual torque are incorrespondence, the error signal is just sufficient to place the firingangle at the value demanded by the motor torque-speed characteristic atthat speed.

For the case of the present system, in which the induction motor assiststhe primary transmission only at low speeds, a speed-torque demandcharacteristic from which is generated the desired torque signalmentioned above, is illustrated in FIG. 6 by reference numerals 45 and46. It will be observed that for the very low speed range, the torquerequired by the curve 45 is a constant torque which is equal to themaximum rated torque for the motor. Above a certain speed the electricassist is to provide proportionately less torque; and so there is alinear decrease (illustrated by the line 46) in the output torque as afunction of increasing speed up to synchronous speed, indicated by zeroslip. It will be apparent to those skilled in the art that the inventionis not limited to the particular envelope illustrated; but any desiredsingle-valued function capable of being generated could be used.

Referring now to FIG. 7, a speedto-voltage converter 48 senses theactual speed of the induction motor 20 and generates a voltage signalrepresentative of this speed. A demand torque network 49 receives theoutput signal of the speed-to-voltage converter 48 and simulates thefunction or output signal described by the combined lines 45 and 46 ofFIG. 6. That is, for a low input voltage representative of low motorspeed, the demand torque network 49 produces a maximum output voltage;and for input voltage levels above a threshold representative of theturning point in the combined curve 45-46, the demand torque network 49produces a correspondingly diminished output voltage until it becomeszero at synchronous speed.

A torque-to-voltage converter 50 senses rotor current and generates avoltage signal proportional to it. This signal, of course, isrepresentative of the actual motor torque. A comparison circuit 51receives the output signals of the demand torque network 49 and thetorque-to-voltage converter 50, and generates an error signalrepresentative of the difference between the two for the polarityindicated. This error signal is coupled through a diode 52 and is fedinto a control amplifier 54. The electric assist actuator, which in theillustrated embodiment is a push switch, which may be mounted on thefloor of the tractor (as at 53a of FIG. 1) and actuatable by footactionof the operator, then energizes a start circuit 54a which also feeds thecontrol amplifier 54. The output of the control amplifier 54 energizes atrigger circuit 56 to set the firing angle of the thyristors in theinverter circuit 35 as determined by the error signal.

Thus, the firing angle of the thyristors in the inverter circuit 35 is afunction of a demand for more or less torque. If the demand for anincrease in torque is relatively large, as previously mentioned, thenthe firing angle is shifted toward 90 rather than to 180. When theactual motor torque is equal to the desired torque for the given motorspeed, then the error signal will be reduced to the value to-operate thetrigger circuit 56 at the required firing angle.

DETAILED CIRCUIT DESCRIPTION Turning now to FIG. 8A, the motor 20 isshown to include a stator winding 20a and a rotor winding 20b. The rotorfeeds the rectifier bridge 33 which includes six diodes connected in aconventional three-phase bridge rectifier configuration. Thepreviously-mentioned inductor 34 is connected in series between theoutput of the rectifier 33 and one input of the inverter 35. The outputof the inverter 35 is coupled to the input of the stator 20a of themotor 20 as shown; and the thyristors of the inverter 35 are againdenoted by an associated Roman numeral which also designates the firingorder.

It will be appreciated that each of the thyristors in the inverter 35 aswell as the diodes in the rectifier 33 may be shunted with aconventional RC-series network for limiting the rate of change ofvoltageacross these elements and for preventing false firing and forprotection.

There is a separate trigger circuit associated with each branch or phaseof the inverter network; and these are designated by reference numerals62, 63, and 64 respectively in FIG. 8A. Each of the trigger circuits62-64 is identical in circuit configuration and only one of them will bedescribed in greater detail within.

The previously-mentioned torque-to-voltage converter 50 comprises anarrangement sometimes referred to as a transductor" including twomagnetic cores 66 and 67 and a fullwave rectifier bridge generallydesignated 68 in series with the secondary of a transformer 69. A loadresistor 69a is connected across the secondary terminals of transformer69. The primary terminal of the transformer 69 is energized by alineto-line voltage taken between second and third source lines (i.e.phases B and C). As rectified rotor current increases, the cores 66 and67 begin to saturate on alternate half-cycles of the AC voltage receivedfrom the transformer 69 thereby coupling into the AC circuit oftransductor 50 an AC replica of the rotor current which upon beingrectified by the bridge 68 generates a voltage across a resistor 70proportional to the rotor current.

The previously-described speed-to-voltage converter 48 includes atachometer 48' in FIG. 8B which senses motor speed and generates avoltage proportional to it. The tachometer assembly includes an internalresistor 74 which forms a voltage divider with a resistor 75. The baseof a transistor 76 receives the signal at the junction of resistors 74and 75. The transistor 76 is provided with a collector load resistor 76aand an emitter load resistor 76b.

The terminal of the resistor 75 not connected to the base of transistor76 is connected to the movable arm of a potentiometer 77, one fixedterminal of which is connected to a negative supply bus 78 through afixed resistor 770. A positive supply bus is designated 79; and a commonbus is designated 80. The potentiometer 77 (along with current limitingresistors 77a and 77b in series) is connected between the negative bus78 and the common bus 80 to provide a reference for the signal fed intothe base of the transistor 76.

The signal taken at the collector of the transistor 76 is fed directlyto the base of a second transistor 76 is fed directly to the base of asecond transistor 83 having a potentiometer 84 in its emitter circuit.The signal taken from the movable arm of the potentiometer 84 is thus aninverted or decreasing function of the signal fed into the base of thetransistor 76 from the tachometer 48'. This circuitry generates thelinear portion 46 of the demand torque characteristic of FIG. 6; and thesetting of the potentiometer 84 varies the slope of this portion 46 asrequired by the characteristics of the motor being used. Hence, thesystem incorporates the capability of translating the juncture of thecurves 45 and 46 (that is, with increase or decreased speed) by a meresetting of the potentiometer 84.

A resistor 85 in series with the fixed terminals of a potentiometer 86is connected from the positive bus 79 to the common bus 80; and a diode87 is connected with its cathode coupled to the movable arm to thepotentiometer 86 and its anode receives the output voltage from thepotentiometer 84 I through a resistor 87a which serves to clip thevoltage fed to the diode. The combination of the diode 87 and thepotentiometer 86 serves as a signal-limiting network which defines anupper limit beyond which the output signal taken from the potentiometer84 may not exceed. Thus, the potentiometer 86 defines the traction limitset point, i.e. the curve 45 of FIG. 6 having zero slope. Thus, thesignal which appears at the anode of the diode 87 is also the signalwhich appears at the output of the demand torque network 49 of FIG. 7and is fed into the comparison circuit 51 thereof.

The comparison means 51 includes a conventional differential amplifier88 receiving, at its positive input through an input resistor 880, thesignal (from lines X-X of FIG. 8A) generated across the potentiometer 70of the torque-to-voltage converter 50. A signal return resistor 88d isconnected between the common bus and the positive input to amplifier 88.The negative one of the leads X--),( is connected to the common bus 80.The differential amplifier 88 receives at its negative input through aninput series resistor 88a, the signal at the anode of the diode 87. Aresistor 88b connected between the other terminal of resistor 88a andthe common bus 80 serves as a signal return resistor for the negativeinput. The output of the differential amplifier 88 is thepreviouslydescribed error signal representative of the differencebetween a desired torque for the instant motor speed and its actualtorque. A feedback resistor 88e is connected between the output ofamplifier 88 and its negative input terminal to set the gain.

The output of amplifier 88 is coupled through a diode 52 (previouslyidentified in the description accompanying FIG. 7) and a resistor 89a tothe positive input of a second differential amplifier 89. A resistor 89bis connected between the cathode of diode 52 and the common bus 80 toset the output impedance of the amplifier 88-diode 52 circuit; and aresistor 890 is connected between the'positive input of amplifier 89 andthe common bus 80 to define the gain into the positive input terminal.The differential amplifier 89 comprises the previously-described controlamplifier 54 of the block diagram of FIG. 7. A feedback resistor 89e isconnected between the output terminal of amplifier 89 and its negativeinput terminal to set its gain. 7

The negative input terminal of the differential amplifier 89 is receivedthrough a series resistor 89d from a resistor-capacitor networkincluding a resistor 90 and capacitor 91 connected to the common bus 80.The other side of the capacitor 91 is connected through normally closedcontacts 92 (actuated by a relay coil 93) and a variable resistor 94 tothe negative bus 78. This network is the start-circuit 54a of FIG. 7.The coil 93 is connected in series with the actuator switch 53 and a DCsupply voltage, as shown. When the actuator switch 53 is depressed bythe operator (that is, it is closed), current flows through the coil 93to open the contacts 92. When the switch 92 is closed, the capacitor 91is charged to a relatively negative potential and the output of theamplifier 89 is positive. However, when the switch 92 is opened byclosing the electric assist actuator switch 53, the capacitor 91 willdischarge through resistor 90, and the negative input of the summingamplifier 89 will return to ground potential. At the same time, theamplifier 89 will amplify the error signal coupled through the diode 52.

The output signal of the amplifier 89 is coupled through avoltage-clipping resistor 89f; and, if positive, the output is limitedby means of a Zener diode 95 and this signal is clamped to ground ifnegative by a diode 96. The output of amplifier 89 is coupled throughresistor 89f to the base terminal of three transistors 97, 98, and 99which act as isolation amplifiers in driving respectively thepreviously-mentioned trigger circuits 62, 63, and 64. The transistors97-99 are connected in the emitter follower configuration; and each hasan emitter load resistor 97a, 98a, and 99a respectively and a bypasscapacitor 97b, 98b, and 99b respectively. The positive voltage from theamplifier 89 is limited by the Zener diode 95 to that necessary toprovide complete retarding of the firing angle of the thyristors in theinverter to 180 which will reduce the rotor current to zero. A positivevoltage (6v. is cutoff) at the output of the amplifier 89 will increasethe firing angle (that is, move it closer toward 180); and zero outputvoltage produces inverter switch conduction at a 90 firing angle.

The output signal of each of the transistors 97-99 is used to define thepositive voltage which biases the cathode of thyristors in the triggercircuit and defines their firing angle relative to the time baseprovided by, for example, phase A of the alternator. When the gatepotential of the associated thyristors is raised above the cathodepotential established by one of the transistors 97-99, then thetriggering thyristor will conduct. Hence, as the cathode potential israised and lowered by its associated transistor isolation amplifier, thefiring angle is varied.

Since all of the trigger circuits 62-64 are similar, only one will bedescribed in greater detail, it being understood that the referencevoltage for each is as shown in FIG. 8B.

Turning now to trigger circuit 63 of FIG. 8C, a transformer, generallyidentified by reference numeral 103, has its primary terminal energizedby the phase A-to-neutral signal; and there are two secondary windingsassociated with the transformer 103, designated respectively 104 and105. The secondary winding 104 has a center tap which receives theoutput signal of transistor 98, and is hereinafter referred to as theinput or control terminal of the trigger circuit.

As previously indicated, the trigger circuit 63 is adapted to triggerthyristor 111 and thyristor VI of the inverter 35 at a phase differenceof 180. Toward this end, a first pulse transformer, generally designated106, has its secondary winding 107 connected through a resistor 108 anda diode 109 across the gate-cathode junction of thyristor Vl. The pulsetransformer 106 has a first primary winding 110 and a second primarywinding 111. A diode 112 and capacitor 113 are connected from oneterminal of the secondary 104 of transformer 103 to the control input ofthe trigger circuit; and the junction between the diode 112 andcapacitor 113 is connected directly to one tenninal of the primarywinding 110 of the transformer 106. The other terminal of the primarywinding 110 is connected to the anode of a thyristor 116; and thecathode of the thyristor 116 is connected to the trigger control input.-The other secondary 105 of the transformer 103 is connected in serieswith a rectifying bridge, generally designated 117, an inductor 118 anda diode 119 to the gate lead of the thyristor 116. The output of thebridge circuit 117 is also connected to a capacitor 120, the other sideof which is connnected to the common input bus 80. A diode 121 isconnected across the capacitor with its anode connected to the commonbus 80.

The other terminal of the secondary winding 105 of the transformer 103is connected to a resistor 122; and the re sistor 122 is connnected tothe cathode of a diode 123 and one terminal of a capacitor 124. Theanode of the diode 123 and the other terminal of the capacitor 124 areconnected to the common input bus 80. The terminal of the secondarywinding 104 of the transformer 103 which is connected to the anode ofdiode 112 is also connected to the anode of a diode 125; and the cathodeof the diode 125 is connected to one terminal of the second primarywinding 111 of the pulse transformer 106. The other terminal of thewinding 111 is coupled to the input control terminal of the triggercircuit through a variable resistor 126.

The trigger circuit for thyristor Vl is identical to that for thyristorVl except that the output pulses of the two are l80 out of phase; and itincludes a pulse transformer having a secondary winding 131 which iscoupled to thyristor l through a'resistor 132 and diode 133. Thetransformer 130 has a first primary winding 134 and a second primarywinding 135. A terminal of the first secondary winding 104 oftransformer 103 is connected through a diode 136 to one terminal of thewinding 134; a capacitor 136a couples this terminal to the control inputof the trigger circuit. The other terminal of the winding 134 isconnected to a thyristor 137. The gate lead of the thyristor 137 iscoupled to the cathode of the diode 123 through another diode 138 withthe cathode of the diode 138 connected to the gate of the thyristor 137.The cathode of the thyristor 137 is connected to the input terminal ofthe trigger circuit.

A diode 139 has its anode connected to the anode of the diode 136 andits cathode connected to one terminal of the second primary winding ofthe pulse transformer 130. The other terminal of the primary winding 135is connected to the control input terminal of the trigger circuitthrough a variable resistor 140.

Since the operation of each side or section of the trigger circuit 63 isthe same, only that portion of the circuit concerned with firingthyristor 116 will be described in detail. it will be understood thateach section of all of the trigger circuits operate similarly except fora phase difference which is caused by the voltage chosen to excite thetransformer. The primary winding of the transformer 103 is excited bythe phase A voltage. It will be appreciated that even though the firingangle, a,

is referred against the line-to-line voltage, this voltage bears aconstant phase relationship to the line-to-neutral voltage; and soeither of these voltages may equally be employed as a trigger referenceas long as an adjustment for the phase difference is made.

The capacitor 120 is provided with a constant charging current throughthe secondary winding 105 of the transformer 103, the full-waverectifying bridge 117, the inductor 118, the diode 123, and the resistor122. This charging current produces a linear voltage ramp acrosscapacitor 120; and when this voltage is sufficient to forward-bias thediode 119 and trigger the gate lead of the thyristor 116 (the cathode ofwhich is at a potential determined by its associated isolationamplifier), the thyristor 116 will fire. Since the voltage-timerelationship of the ramp waveform is known, and it bears a constantrelation with the phase A voltage, it is used as a time base on which toset the firing angle for thyristor Vl.

Assuming that the dotted primary terminal of transformer 103 ispositive, then the capacitor 120 is charging in the proper direction tofire thyristor 116; and the diode 123 is conducting to prevent theaccumulation of a reverse charge on the capacitor 124 (upon which asimilar ramp voltage is built during the negative half of the samecycle). On the previous half-cycle of input voltage, the capacitor 113had charged through the diode 112; and it now stores charge which willflow through the pulse transformer 106 when the thyristor 116 conducts.This discharge of the capacitor 113 through the pulse transformer 106produces a pulse having a time duration defined by the volt-time area ofthe transformer 106. The slope of the top of the pulse is defined by thereflected impedance from the windings of the transformer 106; and theleading edge of this pulse is determined by the capacitor 113.

The total flux change in the core of the transformer 106 is determinedby the reset current through its primary 111; and this flux changedefines the width of the output pulse which, it will be remembered, is60. Hence, adjustment of the variable resistor 126 will vary the currentthrough the second primary winding 111 of the transformer 106; and it,therefore, may be used to set the output pulse width. The pulsetransmitted to the secondary 107 is coupled to thyristor V1 with thecathode of the diode 109 connected directly to the gate lead thereof,and the other terminal of the secondary winding 107 connected to itscathode.

This trigger circuit allows operation of the system independent of thefrequency of operation. If the supply voltage amplitude is varied withfrequency, for example if the frequency decreases, the voltage at thesecondary terminal of transformer 103 will be reduced thereby decreasingthe current charging capacitor 113 and delaying the firing time ofthyristor 116. It will also widen the output pulse of the transformer106 since the volt-time product remains constant; if the voltage isreduced, the time will expand in direct proportion. The voltage,however, must always be of sufficient magnitude to trigger thethyristor. Additional detailed information concerning this triggercircuit may be obtained from a copending, coowned patent application ofMagnuson, et al. for INVERTER FIRING ClRCUlT filed July 15, 1968, Ser.No. 744,716.

Referring now to the complete circuit diagram as illustrated in FIGS. 8A8C, the overall system operation will be briefly described. The bridge33 rectifies the rotor current; and this current is fed through theinductor 34 to the inverter 35. The output of the inverter is fed backto the stator input which is directly connected to the primary powersupply lines; and the amount of power that is coupled back to its statorinput (that is, the slip power that is circulated within the motor) isdetermined by the firing angle of the thyristors in the inverter 35.When the vehicle slows down beneath some predetermined speed, theoperator presses the electric actuator 53 which opens the contacts 92 inFIG. 8B and the capacitor 91 will discharge through the resistor 90thereby raising the voltage level of the negative input of the amplifier89 from a negative voltage level to ground so that the positive outputsignal from the amplifier 89 decreases. The drive to the isolationamplifiers 9799 is thus reduced; and the trigger circuits 62-64 areexcited to fire their associated thyristors in the inverter at a firingangle which shifts from l toward This firing angle is, of course, set bythe voltage biasing the cathode of thyristor 116 in the trigger circuit63. As thyristor 116 is fired by the ramp voltage building up acrosscapacitor 120, the previously-charged capacitor 113 will dischargethrough the pulse transformer 106 and transmit a pulse to fire itsassociated thyristor Vl.

As the rotor current builds up, the current-to-voltage sensor 50 willgenerate an increasing signal which is fed to the positive input ofamplifier 88. At the same time, as the rotor of the induction motorgains speed, the signal input from the tachometer 48 to the demandtorque network will increase. The signal input to the negative terminalof the amplifier 88 will remain constant until the speed exceeds thetraction limit; it will then decrease toward zero. As the signal on thepositive input lead of the amplifier 88 exceeds the negative signal, theinput signal to the positive terminal of the amplifier 89 increases,thereby increasing the output signals of the isolation amplifiers 97-99which, in turn, increases the bias on the cathode of the thyristors inthe trigger circuits and delays the triggering time. As the motoraccelerates; the rotor current and torque are controlled by thecomparator 88. Consequently, for the speed sensed, the output torque ofthe assist motor is regulated according to a predeterminedcharacteristic or envelope as stored in the shaping network.

. lt will be appreciated that a cascade induction motor may be usedequally as wellas the wound-rotor induction motor described. In thislatter case, the output of the second stator would be fed directly tothe rectifier 33 rather than having the rectifier 33 coupled into therotor circuit. The cascade design eliminates the need for sliprings onthe rotor. Similarly, in the broader aspects of the present invention,the demand torque function generator need not be that which has beendisclosed, namely, one which produces a demand signal which decreaseswith increasing speed linearly after exceeding a certain thresholdspeed. For example, if a similar electrical system were used for theprimary transmission, then the speed-torque function generator shouldpreferably generate a true reciprocal relation corresponding to thewell-known constant horsepower characteristic curve illustrated in FIG.4. The inclusion of this function generator does, however, allow thesystem to be adapted to many different purposes; and it permits matchingthe desired characteristic of the electrical system to the loadrequirement. lt is particularly suitable in the present system whereinthe electrical assist is designed to complement a primary transmission;and, therefore, it must be designed to match these characteristics overa certain speed range.

It will be apparent that an overriding clutch should preferably beinterposed between the rear wheels and the induction motor in thespecific application described so that when the vehicle exceeds thesynchronous speed of the motor, it will disengage automatically.

Another contemplated modification of the illustrated system is that theelectric assist actuator generate a linear or variable signal asdetermined by an operator to set vehicle speed. This signal, togetherwith a signal representative of rectified rotor voltage (which isinversely proportional to motor speed) are used as the two inputs to thecontrol amplifier 54; and the demand torque signal is then used as anoverride clamping level for the signal set by the operator to define atorque limit beyond which the system will not drive the assist motor forthat speed. Of course, if the rotor voltage is used, provision must bemade to account for the above-mentioned inverse relation with rotorspeed.

Having thus described in detail a preferred embodiment of the presentinvention, it will be obvious to persons skilled in the art thatmodifications and equivalent structure other than those alreadydescribed may be substituted for that which has been disclosed withoutdeparting from the principle of the in vention; and it is, therefore,intended that all such modifications' and equivalents be covered as theyare embraced within the spirit and scope of the appended claims.

We claim: 1

I. A system for assisting in driving a vehicle having a' prime moversupplying power to first wheel means, said system supplying power tosecond 'wheel means and comprising: an alternator driven by said primemover; an induction motor having a stator energized by said alternatorand a rotor; rectifier means connected in circuit with said rotor forrectifying the rotor current; means including an inverter connected incircuit with said rectifier for coupling rotor current back to saidstator, said inverter including triggerable switching means; and controlmeans including first means sensing motor speed for generating a signalrepresentative of a desired torque for the sensed speed, second meansassociated with said motor for generating a signal representative of theactual torque of said motor, and trigger circuit means coupled to saidfirst and second means for advancing the conduction'angle of saidinverter switching means when said desired torque signal is greater thansaid actual torque signal thereby to increase rotor current.

2. The system of claim 1 wherein'said trigger circuit means includes acomparison circuit receiving said actual torque signal and said desiredtorque signal for generating an error signal representative of thedifference between the same, and pulse generating circuit means forestablishing the conduction angle of said switching means of saidinverter in response to said error signal, whereby said conduction anglewill advance toward 90 when said error signal indicates the desiredtorque is greater than the actual torque and said conduction angle willbe retarded toward l80 when said error signal indicates the desiredtorque is less than the actual torque of said motor.

3. The system of claim 1 further including actuatable means for engagingsaid assist system; said assist system characterized by driving saidsecond wheel means to accelerate said induction motor according to apredetermined speed-torque envelope store in said first means of saidcontrol means.

4. The system of claim 3 wherein said first means of said control meansgenerates a constant desired torque signal representative of a maximumtorque for said motor when said sensed motor speed is below a secondpredetermined speed, and a desired torque signal decreasing with speedwhen said sensed speed is above said second predetermined speed.

5. The system of claim 4 wherein said trigger circuit means includes acomparison circuit receiving said actual torque signal and said desiredtorque signal for generating an error signal representative of thedifference therebetween, and pulse generating means for each of saidinverter switches for advancing the conduction angle thereof in responseto an increase in said error signal thereby to increase the averagerotor current. a

6. The system of claim 4 further comprising first circuit means foradjusting the level of said constant desired torque signal and secondcircuit means for adjusting the slope of said signal which is adecreasing function of speed independently of said first circuit means.

7. In combination with a vehicle having a prime mover. a drive systemcomprising; an alternator driven by said prime mover; an induction motorhaving a stator energized by said alternator and a rotor; rectifiermeans connected in circuit with said rotor for rectifying the rotorcurrent; means including an inverter connected in circuit with saidrectifier means for coupling rotor current back to said stator, saidinverter including switching means; and control means including firstmeans associated with said motor for generating a signal representativeof a demand torque. second means associated with said motor forgenerating a signal representative of the actual torque of said motor,and trigger circuit means receiving said demand torque signal and saidactual torque signal to adjust the conduction angle of said inverterswitching means to cause said actual torque signal to equal said demandtorque si nal.

8. In a vehicle drive system including a prime motor driving analternator, the improvement comprising: an induction motor having astator energized by said alternator and a rotor; rectifier means incircuit with said rotor for rectifying rotor current; inverter meansincluding control switching means for coupling the rectified rotorcurrent to said stator; second circuit means including means sensing thespeed of said motor for generating an electrical signal representativeof said speed; demand torque circuit means receiving said signalrepresentative of motor speed for generating therefrom a demand torquesignal representative of a desired torque; fourth circuit means sensingrotor current for generating an electrical signal representative ofactual rotor torque; comparison circuit means receiving said demandtorque signal and said actual torque signal for generating an errorsignal representative of the difference between the two received signalsand indicative of which of the two signals is greater; and controlcircuit means responsive to said error signal to control the switchingof said inverter switching means to bring the actual rotor torque closerto the demand torque.

9. The drive system of claim 8 wherein said demand torque circuit meansis characterized in that it generates a demand torque signal whichdecreases over at least a portion of the characteristic as the speed ofthe motor approaches synchronous speed.

10. The system of claim 9 wherein said demand torque circuit means isfurther characterized by limiting the value of the demand torque signalto a signal representative of the rated torque of the motor over aportion of the characteristic from zero speed to a speed above aboutone-fourth of synchronous speed.

1. A system for assisting in driving a vehicle having a prime moversupplying power to first wheel means, said system supplying power tosecond wheel means and comprising: an alternator driven by said primemover; an induction motor having a stator energized by said alternatorand a rotor; rectifier means connected in circuit with said rotor forrectifying the rotor current; means including an inverter connected incircuit with said rectifier for coupling rotor current back to saidstator, said inverter including triggerable switching means; and controlmeans including first means sensing motor speed for generating a signalrepresentative of a desired torque for the sensed speed, second meansassociated with said motor for generating a signal representative of theactual torque of said motor, and trigger circuit means coupled to saidfirst and second means for advancing the conduction angle of saidinverter switching means when said desired torque signal is greater thansaid actual torque signal thereby to increase rotor current.
 2. Thesystem of claim 1 wherein said trigger circuit means includes acomparison circuit receiving said actual torque signal and said desiredtorque signal for generating an error signal representative of thedifference between the same, and pulse generating circuit means forestablishing the conduction angle of said switching means of saidinverter in response to said error signal, whereby said conduction anglewill advance toward 90* when said error signal indicates the desiredtorque is greater than the actual torque and said conduction angle willbe retarded toward 180* when said error signal indicates the desiredtorque is less than the actual torque of said motor.
 3. The system ofclaim 1 further including actuatable means for engaging said assistsystem; said assist system characterized by driving said second wheelmeans to accelerate said induction motor according to a predeterminedspeed-torque envelope store in said first means of said control means.4. The system of claim 3 wherein said first means of said control meansgenerates a constant desired torque signal representative of a maximumtorque for said motor when said sensed motor speed is below a secondpredetermined speed, and a desired torque signal decreasing with speedwhen said sensed speed is above said second predetermined speed.
 5. Thesystem of claim 4 wherein said trigger circuit means includes acomparison circuit receiving said actual torque signal and said desiredtorQue signal for generating an error signal representative of thedifference therebetween, and pulse generating means for each of saidinverter switches for advancing the conduction angle thereof in responseto an increase in said error signal thereby to increase the averagerotor current.
 6. The system of claim 4 further comprising first circuitmeans for adjusting the level of said constant desired torque signal andsecond circuit means for adjusting the slope of said signal which is adecreasing function of speed independently of said first circuit means.7. In combination with a vehicle having a prime mover, a drive systemcomprising; an alternator driven by said prime mover; an induction motorhaving a stator energized by said alternator and a rotor; rectifiermeans connected in circuit with said rotor for rectifying the rotorcurrent; means including an inverter connected in circuit with saidrectifier means for coupling rotor current back to said stator, saidinverter including switching means; and control means including firstmeans associated with said motor for generating a signal representativeof a demand torque, second means associated with said motor forgenerating a signal representative of the actual torque of said motor,and trigger circuit means receiving said demand torque signal and saidactual torque signal to adjust the conduction angle of said inverterswitching means to cause said actual torque signal to equal said demandtorque signal.
 8. In a vehicle drive system including a prime motordriving an alternator, the improvement comprising: an induction motorhaving a stator energized by said alternator and a rotor; rectifiermeans in circuit with said rotor for rectifying rotor current; invertermeans including control switching means for coupling the rectified rotorcurrent to said stator; second circuit means including means sensing thespeed of said motor for generating an electrical signal representativeof said speed; demand torque circuit means receiving said signalrepresentative of motor speed for generating therefrom a demand torquesignal representative of a desired torque; fourth circuit means sensingrotor current for generating an electrical signal representative ofactual rotor torque; comparison circuit means receiving said demandtorque signal and said actual torque signal for generating an errorsignal representative of the difference between the two received signalsand indicative of which of the two signals is greater; and controlcircuit means responsive to said error signal to control the switchingof said inverter switching means to bring the actual rotor torque closerto the demand torque.
 9. The drive system of claim 8 wherein said demandtorque circuit means is characterized in that it generates a demandtorque signal which decreases over at least a portion of thecharacteristic as the speed of the motor approaches synchronous speed.10. The system of claim 9 wherein said demand torque circuit means isfurther characterized by limiting the value of the demand torque signalto a signal representative of the rated torque of the motor over aportion of the characteristic from zero speed to a speed above aboutone-fourth of synchronous speed.